The field of the disclosure relates generally to electric power converter equipment and, more particularly, to a system and method for operation of multilevel converters.
Many known multilevel power converters are in use throughout various industries and for a variety of purposes for electric power conversion. Such industries include, without limitation, metals, mining, power, water, oil, and gas.
Specifically, the term “multilevel converter” refers to a converter that can operate in an inverter mode and in a rectifier mode. Some known devices that use multilevel converters include a separated power conversion assembly, or system, electrically coupled to an alternating current (AC) power source, e.g., a utility power grid. Such known separated power conversion assemblies include a rectifier portion that converts the AC transmitted by the utility power generation grid to direct current (DC) and an inverter portion that converts the DC to AC of a predetermined frequency and voltage amplitude. The rectifier portion and the inverter portion use multilevel power converters that may shift between operating as a rectifier and operating as an inverter. Most known multilevel converters include semiconductor-based switching devices, e.g., integrated gate-commutated thyristors (IGCTs) or insulated gate bipolar transistors (IGBTs). The rectifier and inverter portions are typically electrically coupled via a medium voltage DC (MVDC) or a high voltage DC (HVDC) link.
Various known multilevel converter topologies are in service or have been available for service. Many of the DC links for known multilevel converters include capacitors to facilitate levelizing DC voltage within the DC link to stabilize power transmission between the multilevel converters. These capacitors are referred to as “flying capacitors.” The voltages of the flying capacitors vary throughout operation of the associated multilevel converters as the operation of the switching devices in the converter vary. Also, the output voltage pattern and the blocking voltage of each switching device are determined by the flying capacitor voltages. In order to get the appropriate multilevel output with low harmonic distortion and prevent the devices from attaining overvoltage conditions, the flying capacitor voltages are maintained at or near certain voltage levels, which are normally defined as references, or reference voltages for the flying capacitors through all modes of operation of the multilevel converters, including startup. However, in many known multilevel converter topologies, e.g., nested neutral point piloted (NPP) converter topologies there are a large number of switching devices and flying capacitors charging and discharging substantially simultaneously. Therefore, the voltages of the large number of flying capacitors may not be balanced thereby resulting in distortion in the multilevel pulse pattern in the output voltage waveforms at the output of the converter. As such, filtering devices are required at the output of the converters to filter out the distortions in the voltage waveforms. The filters increase the costs of assembling and maintaining the multilevel converters.
Some known multilevel converters use phase-shifted pulse width modulation (PWM) features to facilitate balancing the flying capacitor voltages. However, this method requires additional hardware, which drive up the cost of the converters, with marginal positive results. Also, some known multilevel converters use software-based solutions that facilitate reducing a difference between the real-time capacitor voltages and the associated reference voltages. These solutions are computer resource intensive due to the large number of devices and the speed of the power conversion process, thereby resulting in complicated control mechanisms.